Hard disc drives (HDDs) typically comprise one or more magnetic media discs, each disc having concentric data tracks for storing data. Where multiple discs are used, a stack is formed of co-axial discs having generally the same diameter. A transducing head carried by a slider is used to read from and write to a data track on a disc. The slider is carried by a head arm assembly (HAA) that includes an actuator arm and a suspension assembly, which can include a separate gimbal structure or can integrally form a gimbal. During operation, as the disc spins, the slider glides above the surface of the disc on a small cushion of air. The actuator arm pivots to movably position the slider with respect to the disc. A microactuator assembly can be included to provide additional precision positioning of the suspension assembly. Electrical connections extend along the suspension to electrically connect the transducing head to components located at or near the actuator arm. Those electrical connections can be formed on the suspension itself, or can be located on a separate interconnect structure supported relative to the suspension, such as a flex-on suspension (FOS).
The transducing head typically includes a single writer and a single reader. The reader includes a sensor for retrieving magnetically encoded information stored on the disc (or other magnetic storage media). Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer or layers of the sensor, which in turn causes a change in the electrical properties of the sensor that can be detected by passing a current through the sensor and measuring a voltage across the sensor. Depending on the geometry of the sensor, the sense current may be passed in the plane (CIP) of the layers of the sensor or perpendicular to the plane (CPP) of the layers of the sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover information encoded on the disc.
The writer, for a perpendicular recording transducing head, typically includes a main pole and one or more return poles, which are separated from each other at an air bearing surface (ABS) of the transducing head by gap layers. The main pole and return poles can be connected to each other at a region distal from the ABS by a back gap closer or back via, in some configurations. One or more layers of conductive coils are positioned between the main and return poles, and are encapsulated by insulating layers. To write data to the disc (or other magnetic media), an electric current is applied to the conductive coils to induce a magnetic field in the disc under a pole tip of the main pole. By reversing the direction of the current through the coils, the polarity of the data written to the magnetic media is reversed, and a magnetic transition is written between two adjacent bits. A trailing edge of the main pole is used to write the data to the magnetic media.
Bit patterned media (BPM) systems can be used to store data to a patterned magnetic storage medium (e.g., disc). In a BPM system, data is stored on the disc as discrete magnetic data bits that are isolated from one another. BPM systems can allow for relative high recording densities. However, BPM systems require write synchronization. As the transducing head moves over a surface of the rotating disc, the main pole of the writer must be properly aligned with a selected bit on the disc in order to properly write to the medium. Misalignment can lead to write errors. Therefore, sensors have been proposed for sensing timing marks to synchronize energizing the writer with the arrival of a selected bit at a location adjacent to the main pole of the writer.
Timing variations can negatively affect write synchronization. Known synchronization sensors are typically spaced from the main pole at relatively large distances. For instance, a synchronization sensor that is spaced approximately 6 μm from a main pole of a writer utilized with a disc having a bit length of less than about 20 nm can encompass 300 or more magnetic transitions in the magnetic medium within that space, which tends to increase a risk of synchronization error. In addition, each component in the BPM system can introduce some timing error in making synchronization determinations. For instance, skew angle can exacerbate spacing issues between a synchronization sensor and a main pole of a writer. Variations due to manufacturing tolerances can produce variable spacing of bits on the patterned storage medium. Thermal expansion and other environmental factors can also affect timing variations, such as the thermal effects upon electrical traces that affect signals sent to and from the transducing head to achieve write synchronization. The factors that affect write synchronization could be easily compensated for if the timing variations were deterministic. However, these variations tend to be random, which makes precise synchronization sensing important.
The present invention relates to an alternative apparatus and method for write synchronization with BPM systems.